Disconnect switches are, for the most part, air-break, high-voltage, manually operated switches which are used for isolating high-voltage equipment, so that this equipment may be worked on safely. There are generally speaking, three basic disconnect switches. A no-load disconnect switch is not designed to interrupt any current. Its main purpose is to disconnect equipment after all current flow has been interrupted by some other means. These switches are typically single-pole knife switches which are operated by a hook stick. No-load, air-break disconnect switches are designed to break transformer magnetizing current but not any load current. These switches typically have arcing horns or auxiliary contacts so that sparking will not damage the main contacts. All arcing, during closing or opening, is designed to occur on the arcing horns. Finally, a partial-load, air-break switch is one which is designed to break a specified amount of load current. In addition to arcing horns, they typically have an arc chute or arc box to extinguish quickly the arcing produced by the load current. The present invention may be broadly classified a member of the third group of disconnect switches.
Disconnect switches of the partial-load, air-break variety have, in addition to a main switch blade, an interrupter which is designed to extinguish the arc which results when the switch is opened and to protect the main blade from being damaged by the arc. Preferably, the interrupter should be of the stored-energy variety; that is, they are opened and closed by springs so that the speed of opening or closing is not dependent upon the speed or agility of the person operating the switch. Examples of such switches are illustrated in U.S. Pat. Nos. 2,626,332 to Earle et al; 2,769,063 to Lingal; and 3,778,567 to Davies; German DE No. 3003-713 provides another example.
Knife switches incorporating a spring assist are illustrated in U.S. Pat. Nos. 556,185 to Herrick; 561,581 to Guett; and 655,047 to Bossert.
Those skilled in the art know that in an alternating current circuit, the current and voltage pass through zero amplitude every half-cycle, and that efficient operation of a load break switch is dependent upon the arc path being rendered incapable of supporting re-establishment of the arc after the power current has passed through "zero" following the breaking of the contacts under load. Thus to be effective, an arc extinguishing fluid or gas should be discharged into the arc very rapidly and the arc must be snuffed out to de-ionize the arc path or, at least, to so disperse the ions and breakup of the continuity or density of ionization so that the arc will not restrike during the circuit recovery voltage period.
The puffer interrupter is a superior device for interrupting the flow of load current. In a puffer interrupter, an arc extinguishing fluid, such as SF.sub.6, is puffed into the arc formed at current cut-off by a piston and cylinder combination which is interlocked with the movable contact portion of the interrupter. U.S. Pat. No. 4,123,637 is an example of a high-voltage, high-current air disconnect switch (i.e., 230 KV, 600 Amps) incorporating a puffer-type interrupter to effect circuit interrupting prior to the disconnect switch being fully open.
Those skilled in the art also know that reliable and trouble-free operation is required of a disconnect switch and that a simplified design is preferable over a design which incorporates a multiplicity of springs or a relatively complicated operating mechanism. This is particularly true when it is realized that a disconnect switch is basically a safety device for the protection of operating personal. Therefore, a disconnect switch incorporating a reliable, easy to operate knife switch and an efficient puffer interrupter which does not require extensive maintenance (since it is basically simple in design and construction) and which could function efficiently at lower voltages (1.5 to 35 KV) would be most appreciated by both operating and maintenance personnel. It would go far towards satisfying the electrical industry's need for an improved and inexpensive disconnect switch.